Why does hydrochlorothiazide help treat hydroceles?

No. There is no reason to believe that hydrochlorothiazide helps treat hydroceles. If a patient has scrotal edeam due to fluid overload, then a diuretic such as hydrochlorothiazide may help improve the condition by removing extra fluid from the patient.
It does not. Hydrochlorthiazide is a diuretic & used to control tissue edema, hypertension (can help to control & reduce elevated blood pressure) or control urinary calcium excretion in patients with hypercaluric metabolic stone disease. Does not cause hydrocele fluid to be absorbed. Subject needs to see a urologist if he has a hydrocele large enough to cause discomfort. ? Needs scrotal sonogram to r/o tumor.